The present invention relates to a capacitor having a discharge gap suited for use in a circuit between an antenna and a tuner of, for example, a television set for protecting an antenna input circuit and/or other circuit component parts from the high voltage input brought about by, for example, lightning striking the antenna.
In general, the signal received by a television antenna is transmitted to a television set through a signal transmission line such as, for example, a coaxial cable. In the event that the television antenna is struck by lightning, the high voltage surge of lightning is apt to be impressed on an antenna input circuit with some or all of the circuit component parts being consequently damaged. In order to avoid this, it is known to use in the antenna input circuit a capacitor having a discharge gap defined by a pair of opposed discharge electrodes for causing the discharge of the abnormal high voltage.
One example of the conventional capacitors used for this purpose is illustrated in FIGS. 1 and 2 of the accompanying drawings.
As shown in FIG. 1, the conventional capacitor, generally identified by C, comprises a dielectric substrate 1 having a pair of opposite major surfaces formed with first and second electrode layers 4a and 4b and a common electrode layer 11, respectively, the common electrode layer 11 on one of the opposite major surfaces being so positioned and so sized as to have a pair of opposite portions thereof aligned with the first and second electrode layers 4a and 4b on the other of the opposite major surfaces whereby the capacitor C can represent an electric equivalent circuit wherein, as shown in FIG. 2, two capacitance elements C1 and C2 are connected in series with each other. At respective portions of the first and second electrode layers 4a and 4b generally opposite to those portions thereof which are aligned with the common electrode layer 11, the first and second electrode layers 4a and 4b have respective electrode extensions extending outwardly therefrom and shaped so as to confront each other, leaving a gap g therebetween, which electrode extensions serve as discharge electrodes 5a and 5b.
The conventional capacitor C shown in FIG. 1 is so designed that, when a high voltage is applied between the terminals of the capacitor C, a spark discharge can take place in the gap g between the discharge electrodes 5a and 5b, forming respective parts of the first and second electrode layers 4a and 4b, to protect the other circuit component parts against such high voltage.
In the illustrated conventional capacitor of the construction described above, not only because the capacitance as a whole is derived from the series-connected capacitance elements C1 and C2 as shown in FIG. 2, but also because the first and second electrode layers 4a and 4b forming the respective capacitance elements C1 and C2 together with the common electrode layer 11 have the respective discharge electrodes 5a and 5b formed integrally therewith, the surface area of each of those portions of the first and second electrode layers 4a and 4b which are aligned with the common electrode layer 11 on one side of the substrate 1 opposite to the electrode layers 4a and 4b is necessarily limited. Therefore, unless a dielectric substrate 1 of increased size is used to increase the surface area of each of those portions of the first and second electrode layers 4a and 4b, the capacitance available for a given size is limited.
In addition to the above described disadvantage, there is another disadvantage inherent in the conventional capacitor. Namely, since the first and second electrode layers 4a and 4b are formed on the same major surface of the substrate 1 by the use of, for example, a circuit printing technique, a surface discharge is liable to occur over the major surface of the substrate 1 even at a relatively low voltage and, therefore, the voltage at which discharge is initiated cannot be increased to a high value except with difficulty.